A rack mounting assembly for supporting a third rack assembly within a tub of a dishwasher appliance is provided. The tub defines a wash chamber and a plurality of apertures spaced apart along a transverse direction. The rack mounting assembly includes a mounting body positioned inside the wash chamber and a latching arm positioned outside the wash chamber. The mounting body and/or latching arm include complementary features that pass through the plurality of apertures defined in the tub for engaging each other and securing the mounting body to the tub.